1. Field of the Invention
The present invention relates to a conductive cap for covering an electronic element such as a piezoelectric element mounted on the upper surface of a substrate, an electronic component including such a conductive cap, and a method of forming an insulating film of the conductive cap.
2. Description of the Related Art
For conventional electronic component elements such as piezoelectric resonators or other suitable components, a package structure including a conductive cap has been used. Such a type of package structure will be described with reference to FIG. 6.
The package structure contains a substrate 51 having a rectangular sheet shape, made of an insulating material such as alumina, and a metallic cap 52. On the upper surface 51a of the substrate 51, terminal electrodes 53 and 54 for achieving external electric connection are provided. The terminal electrodes 53 and 54 each are extended onto the side surfaces, the end surfaces, and the lower surface, in addition to the upper surface 51a, so that the package structure can be surface-mounted onto a printed circuit board.
Further, a piezoelectric element is mounted onto the upper surface 51a of the substrate 51 by solder, a conductive bonding agent such as a conductive adhesive, or the like, though not shown in FIG. 6. The electrodes of the piezoelectric element are connected to the terminal electrodes 53 and 54, respectively.
For the purpose of sealing the piezoelectric element, a metallic cap 52 having an opening at the lower portion thereof is bonded to the substrate 51. A rectangular frame-shaped insulating film 55 is provided on the portion of the upper surface 51a of the substrate 51 that is arranged to be contacted with the end surface of the opening of the metallic cap 52. The insulating film 55 is formed by printing synthetic resin, or printing and baking glass. The thickness of the insulating film 55 is about 0.1 mm. Accordingly, the height of the chip electronic component to be produced can be reduced, even though the metallic cap 52 is used.
In recent years, it has been required to reduce the mounting areas of electronic components. With even greater miniaturization of components, it has been very difficult to print and form the insulating film 55 on the upper surface 51a of the substrate 51 with high precision.
Further, Japanese Unexamined Patent Application Publication No. 6-132762 discloses an electronic component 71 having a cap shown in FIG. 7. In this electronic component 71, terminal electrodes 73 and 74 each extends on the upper surface, the side surfaces, and lower surface of a substrate 72 made of an insulating material. A piezoelectric element 75 is bonded to the terminal electrodes 73 and 74 via solder members 76 and 77.
A metallic cap 78 is bonded to the upper surface of the substrate 72 by an insulating adhesive. In order to prevent the terminal electrodes 73 and 74 and the metallic cap 78 from short-circuiting, an insulating film 79 is disposed on the entire inner surface of the metallic cap 78.
In the prior art, no method of forming the insulating film 79 is specifically described. It is presumed that the insulating layer is precoated on a sheet material before the material is shaped into the metallic cap 78, and the precoated sheet material is formed to have the same shape as that of the cap.
When the metallic cap described in Japanese Unexamined Patent Application Publication No. 6-132762 is produced, the peripheral portion of the opening of the metallic cap 78 must be bent outward, as shown in FIG. 7, so that the insulating layer is always positioned on the portions of the terminal electrodes 73 and 74 to be contacted with the metallic cap 78. Accordingly, the process of producing the metallic cap 78 becomes complicated. Further, in some cases, when such bending work is carried out, the insulating layer 79 is peeled, making it impossible to reliably provide electrical insulation between the metallic cap 78 and the terminal electrodes 73 and 74.
In a chip electronic component in which an element such as a piezoelectric element or other electronic element is mounted onto a substrate, the piezoelectric vibration portion thereof is vibrated. As a result, the structure is such that the element is accommodated in a package that does not interfere with vibration of the element. After the element is mounted onto the surface of the substrate, a cap is fixed to the surface of the substrate to cover and seal the element.
FIG. 20 is a cross-sectional view showing an example of another conventional electronic component sealed with such a cap. In the conventional electronic component shown in FIG. 20, an insulating cap 30 is used. This is because the cap 30 is fixed to terminal electrodes 11 and 12 provided on a substrate 10 and must be insulative in order to prevent the terminal electrodes 11 and 12 from short-circuiting. An element such as a piezoelectric resonator is mounted onto the upper surface of the substrate 10. The element 20 is bonded to the terminal electrode 11 via solder 21 and the terminal electrode 12 with solder 22. A terminal electrode 13 is provided between the terminal electrodes 11 and 12 between the terminal electrodes 11 and 12 on the lower surface of the substrate 10 to produce a capacitor between the terminal electrodes 11, 12 and 13.
As the insulating cap 30, a ceramic cap, a resin cap, or other cap is used. However, these caps must have a thickness of 0.25 mm or larger because of the forming capabilities and limitations. Accordingly, it is not possible to reduce the height of the electronic component, and the area of the substrate becomes larger.
For the purpose of reducing the height of the electronic component and enhancing the integration density of the circuit board, it is preferable to use a metallic cap. However, if the metallic cap alone is fixed to the substrate, the terminal electrodes short-circuit, as described above.
In order to prevent the terminal electrodes from short-circuiting as described above, a method in which an insulating layer 31 is disposed on the substrate 10 and the terminal electrodes 11 and 12 onto which the metallic cap 32 is to be mounted, and the cap 31 is mounted onto the insulating layer 31, as shown in FIG. 21. In FIG. 21, elements such as a piezoelectric resonation element mounted onto the substrate 10 are omitted.
According to a method as described above, the metallic cap can be used. Thus, the height of an electronic component can be reduced. On the other hand, there arises the problem that with further miniaturization of the electronic component, it becomes more difficult to form the insulating layer on the substrate with high precision.
Japanese Unexamined Utility Model Application Publication No. 62-158828 and Japanese Unexamined Patent Application Publication No. 8-204491 disclose an electronic component provided with a metallic cap that is made of an aluminum sheet having an anodized film on the surface thereof. However, for the metallic cap, a sheet material is anodized. Therefore, in the case where the sheet material is formed into a cap shape, the end surfaces of the cap to be contacted with terminal electrodes on a substrate have no anodized films thereon. Accordingly, the terminal electrodes can not be prevented from short-circuiting.
Moreover, Japanese Unexamined Patent Application Publication No. 6-132762 discloses a metallic cap provided with an insulating layer disposed on the inner surface thereof. Such a metallic cap is shaped so that the end portions of the cap to be contacted with a substrate are bent outward, and the inner surface of the cap can be contacted with terminal electrodes. Accordingly, the insulating layer disposed on the inner surface is contacted with the terminal electrodes. Thus, the insulating layer is interposed between the metallic cap and the terminal electrodes, whereby the terminal electrodes can be prevented from short-circuiting.
In Japanese Unexamined Patent Application Publication No. 6-132762, a method of forming the insulating layer is not specifically described. It is presumed that the insulating layer is precoated on a sheet material before the material is formed in the metallic cap shape, and the precoated sheet material is formed into the cap shape. In the case of such a metallic cap, the end portions of the cap are bent outward so that the insulating layer on the end portions of the cap is positioned on the portions of the cap to be contacted with the terminal electrodes. Further, during bending, the insulating layer may be peeled so that the insulating properties become insufficient.
In some cases, it is necessary to provide an insulating layer for the outer surface of the metallic cap, so that insulation between the electronic component and other elements is maintained when the electronic component is mounted onto a circuit board. In Japanese Unexamined Patent Application Publication No. 6-132762, only a method of forming the insulating layer on the inner surface of the cap is disclosed.